1. Field of the Invention
The present invention relates to an electrode for an electric double-layer capacitor, and a slurry for forming the electrode.
2. Description of the Related Art
There is a conventionally known electric double-layer capacitor made using a solution of quaternary ammonium borofluoride compound in propylene carbonate as an electrolyte. Vapor-activated carbon having a turbulent structure is used as activated carbon in electrodes of the electric double-layer capacitor, namely, positive and negative electrodes.
With the electric double-layer capacitor provided with the positive and negative electrodes made from the activated carbon described above, a relatively large electrostatic capacity can be provided, but in order to meet the demand as an energy source for a hybrid-type automobile, for example, a further increase in electrostatic capacity is desired.
To meet such demand, the present inventors have made various studies, and as a result, they have reached the consideration of alkali-activated carbon made from meso-phase pitch as a starting material for use as activated carbon for the positive and negative electrodes. The alkali-activated carbon has a graphite structure of a good electric conductivity incorporated in a surface of particles of the activated carbon, thereby realizing the maintaining of a high density and a low resistance. With the electric double-layer capacitor including the positive and negative electrodes made using such alkali-activated carbon the electrostatic capacity can be increased remarkably more than that in the conventionally known capacitor.
However, as a result of studies further advanced, it has been found that the rest potential of the alkali-activated carbon in an electrolyte is offset relatively largely toward an oxidizing reaction region from an ideal rest potential of the activated carbon, and for this reason, there is a possibility that an oxidizing reaction may be liable to occur in the positive electrode during charging, whereby the durability of the electric double-layer capacitor may be impeded.
There is a conventionally known electrode for an electric double-layer capacitor, which includes an active material and a conductive material and is connected to a current collector.
When the resistance of contact between the current collector and the electrode is high, an increase in internal resistance of the electric double-layer capacitor is brought about. Therefore, to decrease the contact resistance, for example, the following measures are conventionally employed: a measure of subjecting a current collector made of an aluminum foil to an etching treatment to increase the contact area, and a measure of bonding an electrode to a current collector by an adhesive containing a conductive filler and then subjecting the collector and the electrode to burring or punching to enhance the close contact between the collector and the electrode.
However, the conventional measures are incapable of decreasing the contact resistance to an expected extent even with a high working cost. Therefore, a further decrease in contact resistance is desired.
Further, there is a conventionally known electrode for an electric double-layer capacitor, which is formed using a slurry under utilization of a doctor blade process.
Such a slurry conventionally contains CMC (sodium carboxymethyl cellulose) added as a thickening agent in addition to activated carbon, a conductive filler and a binder.
It may be contemplated that activated carbon made from meso-phase pitch as a starting material, i.e., meso-phase activated carbon is used as the above-described activated carbon to aim at an increase in electrostatic capacity. However, the following problem is encountered: This meso-phase activated carbon is liable to be agglomerated in a slurry, because it has a high water-absorbing ability, and when an agglomeration has been produced, the thickness and density of the formed electrode are non-uniform, resulting in decreases not only in performance but also in strength of the electrode.
Accordingly, it is an object of the present invention to provide an electrode of the above-described type which is formed using a particular material including the above-described alkali-activated carbon as a main component and which is capable of increasing the electrostatic capacity of an electric double-layer capacitor of the above-described type and enhancing the durability of the capacitor.
To achieve the above object, according to the present invention, there is provided an electrode for an electric double-layer capacitor, which is formed using a solution of a quaternary ammonium borofluoride compound in propylene carbonate as an electrolyte, wherein the electrode includes alkali-activated carbon made from meso-phase pitch as a starting material, and a conductive filler having a rest potential smaller than that of the alkali-activated carbon in the electrolyte, the amount Fc of conductive filler incorporated being in a range of 10% by weightxe2x89xa6Fcxe2x89xa640% by weight.
If the electrode is formed as described above, the rest potential of the alkali-activated carbon can be shifted toward an ideal rest potential for the activated carbon. Thus, it is possible to conduct the charging of the electric double-layer capacitor within a usable potential window of the electrolyte to prevent the occurrence of an oxidizing reaction in a positive electrode.
However, if the amount Fc of conductive filler incorporated is lower than 10% by weight, there is no adding effect. On the other hand, if the amount Fc of conductive filler incorporated is higher than 40% by weight, the electrostatic capacity of the electric double-layer capacitor is decreased suddenly.
It is another object of the present invention to provide an electrode of the above-described type, wherein the resistance of contact between the current collector and the electrode can be decreased by improving the structure of the electrode.
To achieve the above object, according to the present invention, there is provided an electrode for an electric double-layer capacitor, which includes an active material and a conductive material and which is bonded to a current collector, wherein the concentration of the conductive material in a surface portion of the electrode bonded to the current collector is higher than the concentration of the conductive material in an internal portion of the electrode.
If the electrode is formed as described above, a reduction in resistance of the surface portion of the electrode can be realized to decrease the resistance of contact between the surface portion of the electrode and the current collector, thereby decreasing the internal resistance of the electric double-layer capacitor.
It is a further object of the present invention to provide a slurry of the above-described type in which meso-phase activated carbon can be dispersed homogeneously or substantially homogeneously by using particular CMC (sodium carboxymethyl cellulose), and from which an electrode having a high strength can be formed.
To achieve the above object, according to the present invention, there is provided an slurry for forming an electrode for an electric double-layer capacitor, which includes meso-phase activated carbon and CMC, the degree De of etherification of the CMC being in a range of 0.6xe2x89xa6Dexe2x89xa60.9.
If CMC as described above is used, the dispersion property of the meso-phase activated carbon can be improved to provide an electrode strength increasing effect provided by the meso-phase activated carbon and an electrode strength increasing effect exhibited by the CMC itself, namely, by a carboxymethyl group. Therefore, if the slurry is used, it is possible to produce an electrode whose productivity can be enhanced under utilization of a doctor blade process, and which has a high electrostatic capacity and an excellent durability.
However, if the degree De of etherification of the CMC is lower than 0.6, the dispersion property of the meso-phase activated carbon is good, but the strength of the electrode is low, because the amount of carboxymethyl groups is small. On the other hand, if De  greater than 0.9, the amount of carboxymethyl groups is increased and hence, the strength of the electrode must be increased intrinsically. However, the dispersion property of the meso-phase activated carbon is degraded and as a result, the strength of the electrode is decreased.
It is a yet further object of the present invention to provide an electrode which has a high strength and which is produced using particular CMC.
To achieve the above object, according to the present invention, there is provided an electrode for an electric double-layer capacitor, which includes meso-phase activated carbon and CMC, the degree De of etherification of the CMC being in a range of 0.6xe2x89xa6Dexe2x89xa65 0.9.
With the above configuration, it is possible to provide an electrode for an electric double-layer capacitor, which has a high strength.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.